1. Field of the Invention
The present invention relates to an optical writing device, an image forming device including the optical writing device, and an optical writing method.
2. Description of the Related Art
As examples of conventional technologies for writing data with an optical beam are cited what are disclosed in Japanese Patent Application Laid-Open No. 2003-034051 and Japanese Patent Application Laid-Open No. 2001-051214. Japanese Patent Application Laid-Open No. 2003-034051 discloses a technology for correcting the scanning irregularity caused by a scanning lens or the like. According to the technology, a count value of a high-frequency clock with a frequency higher than that of a pixel clock is compared with data on the phase-shift amount of a pixel clock per unit of the high-frequency clock. The phase-shift amount data is used to correct the displacement of the write starting position in a scanning direction due to scanning distance variations from the rotation axis of a reflecting surface of a deflector or displaced luminous points of light fluxes in the scanning direction. Based on the comparison result, at least one of the rising edge and the falling edge of the pixel clock is phase-shift controlled.
Japanese Patent Application Laid-Open No. 2001-051214 discloses a technology for correcting magnification error of an image in a main scanning direction with a simple configuration. In the technology, a temperature sensor detects a temperature of an f-theta lens, and correction data corresponding to the detected temperature is read from a correction amount storage section. A write clock generating section generates a write clock frequency for the correction data. With the write clock frequency, the magnification of the image is changed in the main scanning direction. After the temperature of the f-theta lens is detected and the write clock frequency is corrected, the total magnification of the image is adjusted on a drum.
In addition, Ricoh Technical Report No. 30, December, 2004 discloses operations for expanding and contracting a pixel clock.
In an electrophotographic image forming device such as a digital copier including an optical writing device, even intervals of laser beams as an image forming medium on a photosensitive drum are maintained by the f-theta lens. Recently, a material of the f-theta lens is switched from glass to plastic to reduce costs. However, the plastic f-theta lens may be affected by heat generated in the device. Namely, as the temperature in the device is getting increased, the plastic f-theta lens becomes to expand by the heat. As a result, it may fail to perform the laser beam writing in an exact position on the photosensitive drum. The f-theta lens has the magnification deviation in the main scanning direction so that widths of any areas in the main scanning direction are not equal.
For example, the technology disclosed in Japanese Patent Application Laid-Open No. 2003-034051 is known as the one for solving the problem. In the technology, a position correction in the main scanning direction is performed by expanding and contracting a write clock frequency (CLK) singly and changing the width of the write CLK in any main scanning position of the image, and a laser writing position is adjusted to the target position. When a write starting position of the image is to be shifted to the back-end side of the main scanning position, the width of the write CLK at any position is expanded, and the following main scanning position is also shifted by the expanded distance. By repeating the operation, a laser beam can illuminate anywhere the image is to be formed. Furthermore, the magnification deviation of the f-theta lens is cancelled by expanding and contracting the width of the write CLK based on the deviation in each area.
However, according to the technologies described above, when the CLK is expanded and contracted, a laser beam (pixel concentration information) generated in the CLK is also expanded and contracted. In a color copier, particularly, a color image is formed by a combination of YMC (yellow, magenta, and cyan) colors, and therefore, a color of a portion where the CLK is expanded and contracted differs from that of a portion where the CLK is not expanded and contracted. The color differences may be regarded as image degradation.